Photosynthesis

Photosynthesis

1. structure and role of chloroplasts

   • chloroplasts have a double membrane
   • grana are stacks of up to 100 disc like structures called thylakoids, this is where the LDR takes place and where the photosynthetic pigments are located
   • the stroma is the fluid filled interior where the LIR takes place
   • two main type of photosynthetic pigments: chlorophylls and carotenoids
   • chlorophylls absorb light from red and blue-violet regions and carotenoids absorb light from blue-violet region
   • two main types of caretnoids the carotenes and xanthophylls act as accessory pigments

2. light harvesting

   • the chlorophylls and accessory pigments in the thylakoid membrane are grouped in clusted of several hundred molecules called an antenna complex.
   • photons then reach chlorophyll a which is the primary pigment molecule and known as the reaction centre
   • photosystem I is arranged around a chlorophyll a molecule with an absorption peak of 700nm
   • photosystem II is arranged around a chlorophyll a molecule with an absorption peak of 680nm

3. light dependent reaction

   • 1. in non-cyclic photophosphorylation photons of light are absorbed by PSII and passed to chlorophyll a in the reaction centre
   • 2. causes the displacement of 2 high energy electrons that are raised to a higher energy level when picked up by electron acceptors
   • 3. electron accepts pass electrons though electron transport chain to PSI
   • 4. energy lost by electrons used to convert ADP to ATP
   • 5. photons of light absorbed by PSI and passed to reaction centre, 2 high energy electrons emitted and passed through ETC
   • 6. PSII is unstable as lost electrons so electrons replaced through photolysis
   • 7. second electron acceptor that receives electrons from PSI passes these electrons to hydrogen ions on the outside of the thylakoid membrane where they reduce NADP
   • cyclic photophosphorylation the electrons that aren't taken up in the production of reduced NADP pass along the ETC then returned to PSI, ATP synthesised but not reduced NADP

4. light independent reaction

   • 1. 5C acceptor molecule ribulose biphosphatecombines with CO2 to form unstable 6C, catalysed by enzyme rubisco
   • 2. 6C compound immediately spilts into 2 molecules of glycerate-3-phopshate
   • 3. GP phosphorylated by ATP and reduced by reduced NADP to triose phosphate
   • 4. some of triose phosphate can be built into glucose phosphate and then into starch by condensation
   • 5. for the cycle to continue most of the triose phosphate is regenerated into RUBP, NADP is reformed and goes back to light dependent reaction to be reduced again

5. product synthesis

   • macronutrients needed in large amounts, but micronutrients needed in much smaller amounts
   • nitrogen taken up by roots as nitrates, transported as nitrates in xylem and as amino acids in phloem, used fro synthesis of proteins and nucleic acids, nitrogen defiency can lead to reduced growth and yellowing of leaves due to not enough chlorophyll- chlorosis
   • magnesium absorbed as Mg2+ and function is chlorophyll production and activation of ATPase, required by all tissues but especially leaves and is transported as Mg2+ in xylem
   • magnesium forms part of chlorphyll molecule, chlorosis begins between veins of older leaves and is main symptom of Mg deficiency as existing Mg in plant is mobilised and transported to newly formed leaves